The present invention relates to a coating device for coating a traveling web from a chamber of coating composition. Such a coating device is known from U.S. Pat. No. 4,250,211.
This type of coating device includes a coating composition application space defined between the mouth of the chamber for the coating composition, the doctor element for the composition at the downstream end of the application space and the web of material. The application space is limited on its inlet side with respect to the motion of the web of material by a damming plate, which is so shaped and of a length as to form a throttle slot in cooperation with the web of material, so that a pressure greater than atmospheric pressure can be maintained in the application space. It is beneficial to feed the coating composition to the application space from the chamber or reservoir for the coating composition in an amount considerably in excess over that amount actually used. In the prior art coating device, the excess may amount to about twenty times the amount of material required. Excess coating composition then passes out through the throttle slot along the web of material. This has the advantage that a uniform coat is obtained, particularly if the path between the application region of the coating composition and the point of the doctor where the excess coating composition is scraped off the web of material is of relatively short length.
However, this structure has the disadvantage that a rotary flow of the coating composition takes place in the application space. A large amount of air also accumulates in the center of the eddy. Air periodically leaves the application space through the throttle slot in the form of a plug or bubble of air. Further, the excess coating composition in the rotary flow must find a path, which is transverse to the stream of new or fresh coating composition that is being fed. That path leads to the damming slot located above the damming plate and into the open. This exiting of the composition takes place explosively because of the hydrodynamic pressure produced by the web of material. All of this leads to disturbances in the coating applied. Furthermore, the coating composition which spurts out presents a considerble danger of soiling and dirtying the surrounding parts.